Andrew J Bromage wrote:
This table may help clarify the thinking:
| Algorithm | Codec | Format ----------------+---------------+---------------+---------------- Cipher | DES | CBC-DES | SSL MAC | SHA-1 | HMAC-SHA-96 | SNMPv3 Text compress | LZ77 | Deflate | GZip Image compress | DCT | JPEG | JFIF
These algorithms are all normative. All compliant encoders and decoders produce the same output for the same input (they are true functions with inverses). How would you handle something like the MPEG audio and visual specs which only specify the bitstream, and not the actual algorithm? For example, two MPEG encoders with the same set of parameters can produce two totally different, yet compliant, bitstreams. Decoders, on the other hand, basically produce the same output for the same input (there is allowance for different roundoff/truncation strategies). MPEG is also complicated by the various layers, so there are multiple formats. For example, an MPEG program (a group of audio and video streams) can be stuffed into a program stream (DVD's do this) or a transport stream (digital TV via ATSC or DVB). AVI and QuickTime formats also a have a similar issue, where they really specify a file format, and you can use a variety of codecs in them. This may not be a problem, but it is something to consider. -- Matthew Donadio (m.p.donadio@ieee.org)